The following instructions offer one way to assess the accuracy of published slope ranges for soil map unit polygons.
In the US, the USDA NRCS Soil Survey creates and publishes soil maps. Soil surveys are maps of soil polygons, detailed descriptions, and use/management interpretation tables. “Soil Series” and “Soil Map Units” are defined, in part, by slope (also elevation, bedrock parent material, temperature-moisture category, rangeland/forestland location, etc.). Soil Map Units are the numbered polygons shown on soil maps. A survey area usually contains a few dozen Series. A Soil Series may be composed of multiple Soil Map Unit polygons, thus, several thousand polygons comprise a soil survey area.
Example Soil Survey Area:
Soil Survey of the Colville Indian Reservation, parts of Ferry & Okanogan Counties, WA
Example Soil Series nomenclature (Winchester Series):
WINCHESTER – Mixed, Mesic, Xeric Torripsamments
OSD for Winchester Series
Soil Map Units for Winchester Series:
521 – Winchester loamy coarse sand, 0 to 10 percent slopes
522 – Winchester loamy coarse sand, 10 to 25 percent slopes
523 – Winchester loamy coarse sand, 25 to 60 percent slopes
1.) Acquire Data
– The easiest, most user-friendly way to download soil survey data is through the “SSURGO Data Downloader”, available from ESRI…CLICK HERE. Data is packaged by watershed and comes as a MapPack (.mpk) for use in ArcGIS. If the .mpk file does not open when double-clicked, uncompress it with a program like 7-Zip (or similar). You will get a set of folders. Collect these all in a new folder that you name intelligently. Then look in the “v10” folder for the .mxd. Load it in ArcMap. Bam.
– Download soil survey custom maps and data for specific regions of interest from the online mapping tool at WebSoilSurvey.
– Soil Series descriptions are found at Official Soil Survey Descriptions (OSD).
– Soil GIS layers are available from Geospatial Data Gateway. “SSURGO” data is the most detailed soils info in the US. Usually you download soils data by County, National Forest, or Indian Reservation. The “spatial” data are the shapefiles with limited amount of attribute information. The “tabular” dataset contains physical and chemical soil properties, soil interpretations, and metadata.
– Archived Soil Survey Manuscripts are also available. While the latest (official) soil survey data is found through Web Soil Survey, “archived” copies of older survey versions are found at Web Soil Survey > Archived Soil Surveys or on the shelf at most university libraries. These come as printed volumes or as PDFs. Some local NRCS offices have copies, too.
– Several links to soil-related Tools.
– Download DEMs and clip to your survey area’s boundary polygon (use EarthExplorer or another DEM source).
– Digital Raster Graphics (georeferenced topo maps in .tif format) from LibreMap
– Digital Line Graphs (DLG) shapefiles: HERE
– Optional: Download temperature data from Oregon State’s PRISM Climate Group
– Optional: Download geological GIS data from the Geological Survey or Bureau of Mines for your state:
WA OR ID MT UT NV AZ CA AK SD WY
– Optional: Rangeland vs. Forestland polygons analysis…
– Optional: Land Use/Land Cover analysis…
2.) Get Prepared
– Add layers to a new map in ArcMap.
– Set up Data Frame Coordinate System and Display Units (Ex: UTM NAD83 Zone 11, Meters).
– Grid the survey area in preparation for sampling (Sampling Grid). Maybe 10km x 10km grid cell size? We’ll not assess the entire survey area here, rather just a representative portion of it (certain grid cells).
– Select the grid cell you will use for this analysis.
– Clip soils and base layers to your study area polygon or set of sampling grid cells.
– Create a slope raster in percent rise units (smoothing needed?).
3.) Do the GIS Analysis
– How many different map units are there in your study area? Map units are found in the MUSYM field in the attribute table. MUSYM numbers are the labels used on paper soil maps. Map units in newer surveys use numbers only. Older surveys are letter-number combos.
– Zonal statistics to extract summary statistics for map unit polygons inside sample grid cell (study area)…
4.) Compare Sample Distribution to Published Slope Range
– Create a histogram of extracted slope for each soil map unit (use 1% bins). Compile data as necessary to create one histogram for map units.
– Identify Mean and Median of histogram.
– Add a graphic (or series) showing published slope range for the polygon.
– Write up your results…Where/which soil polygons match best/worst?
– Similar analyses can be performed on elevation, bedrock parent material, temperature-moisture categories, rangeland/forestland, etc.